1. Field of the Invention
The present invention relates to a process for removing diene from ethylene propylene diene monomer (EPDM) resins and more particularly to a process for reducing ethylidene norbornene present in such resins.
2. Description of the Prior Art
The introduction of high activity Ziegler-Natta catalyst systems has lead to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefin polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions. More recently, the gas phase fluid bed reactor has been extended to the production of rubbery resins such as ethylene propylene diene monomer such as disclosed in U.S. Pat. No. 4,710,538 issued Dec. 1, 1987.
Ethylene propylene diene monomer (EPDM) resins are presently produced by a variety of techniques. In certain processes for producing EPDM resins, wherein the diene is ethylidene norbornene, the amount of ethylidene norbornene monomer, remaining in the finished polymer, (i.e., after completion of the devolatilization process to remove dissolved monomer from the polymer,) in a free form is such as to cause objectionable odors.
Thus when producing and recovering EPDM resin according to certain of the above processes, there still exists a somewhat objectionable odor in the resins product. This is primarily due to the fact that ethylidene norbornene (ENB) a monomer used to produce sulfur curable ethylene-propylene rubbers has a very distinctive and objectionable odor and is present in minute quantities in the finished product. Ethylidene norbornene can be detected by humans at concentrations as low as 14 ppbv. For example, in a typical fluidized bed process EPDM resins contain as much as 15% by weight of the dissolved ENB.
Current techniques for reducing the amount of ENB present in EPDM resins produced by a gas fluidized bed process involves counter current purging of the settled resin with nitrogen whereby the recovered ENB is recycled to the reactor. Unfortunately however with this technique, resin sintering occurs and excessive nitrogen usage is required.
More recently, U.S. Pat. No. 4,910,295 issued Mar. 20, 1990 and assigned to a common assignee discloses a process for reducing the amount of ENB present in EPDM resin wherein a crystalline siliceous molecular sieve is introduced into the EPDM resin in an amount sufficient to reduce the odor produced by ENB present in said EPDM resin.
The present invention is based on the discovery that the amount of diene e.g., ethylidene norbornene present in EPDM resin can be substantially reduced by subjecting the resin to microwave energy in the presence of a dielectric material for a time and in an amount sufficient to evolve ENB vapors from the EPDM resin without causing any physical or chemical changes in the EPDM resin, while simultaneously subjecting the EPDM resin to an inert gas, such as nitrogen, to remove vaporized ENB from the resin and to prevent sintering of the resin.